Membrane Transport

Alex Aitken

13 May 2015

1

Objectives

• To understand the role that cellular membranes,

concentration gradients and membrane proteins have on

cells exchanging molecules in their environment

CellMembrane

More than a protection...

Cell Membrane• Protects the cell.

• Allows certain molecules to enter the cell and not others.

• Contains the cytoplasm and organelles.

• Is constituted by a bilayer of phospholipids, with proteins trapped in the membrane.

Membranes are formed of phospholipids.

• Phospholipids are amphipathic molecules: they have in the same molecule hydrophilic and hydrophobic regions

• Phospholipids are the most abundant lipid in membranes.

Phospholipids

• Lipid bilayer – hydrophilic “head” and hydrophobic “tail”

Components of the plasma membrane

“Fluid Mosaic” Model of cell membrane with proteins

In this model, you see how the cytoskeleton attaches to proteins of the membrane

There are also proteins attached to the membrane. They have various functions.

• Membrane proteins– in animals, proteins

constitute about 50% of the mass of the plasma

membrane

Functions of Membrane Proteins

Fig 11-20 Essential Cell Biology

Principles of a selectively-permeable membrane

Small

Hydrophobic

Molecules

O2

CO2

N2

Small

uncharged

polar

molecules

H2O

ethanol

glycerol

Larger

uncharged

polar

molecules

Glucose

Nucleotides

Amino acids

Ions

H+, Na+

Mg2+, K+

Ca2+, Cl-Cell membrane

Membrane permeability depends on:

• Molecule’s lipid solubility

• Molecule’s size

• Lipid composition of the membrane

Principles of a selectively-permeable membrane

• Passive transport:

- No energy is required

- Movement down a concentration gradient

• Active transport:

- Energy is required

- Movement against a concentration gradient

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One way to categorize how molecules move across the

membrane is by their energy requirements

Active and Passive Transport

Passive diffusion: from a high concentration to a low concentration.

Passive Transport- Simple Diffusion

• Diffusion- movement from

an area of high molecule

concentration to low

concentration

• Continues until the

concentration is the same in

all regions- What is this

called?

• Equilibrium

Passive Transport- Facilitated Diffusion

• Facilitated diffusion

– Molecules that cannot cross membrane easily use membrane transport proteins to move down their concentration gradient

– NO ENERGY REQUIRED

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Passive transport: Use a protein as a carrier, or channel, from high concentration to low concentration.

One Direction

Channel protein vs. Carrier protein

Channel protein-

act like doorways in

that are either open or

closed-

Transportation is fast

Carrier protein-

like revolving doors in

that allow movement in

and out of the cell without

creating an open hole

Transportation is slower

Chanel protein- Ion channels

• Ion channels allow the passage of ions such as Na+, K+, Ca2+, and Cl- to pass membrane depending upon chemical gradient

• Channel proteins are either Open channels or Gated channels

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Channel proteins- Ion Gated Channels

• What controls the opening and closing of ion gated

channels?

1. Mechanically gated channels- controlled by a

physical force such as increased temperature or

pressure on the membrane (e.g. auditory hair cells)

2. Chemically gated channels- controlled by biding to

chemical messenger (neurotransmitter)

3. Voltage-gated channels- open and close when the

electrical state of the cell changes

Carrier proteins

• How it works

• Why do you think diabetics lose

glucose in their urine?

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• Molecules that otherwise can’t cross

the membrane use carrier proteins to

help transport both ions and other

solutes across the membrane

• Never form a direct connection

between the intracellular and

extracellular fluid

• Concentration gradient

required, but energy is not

Properties

-Selective for the molecule they

transport such as the GLUT transporter

can only transport glucose across the

membrane

-They can be saturated (i.e. rate of

transport limited by number of

transporters)

Competition occurs between transport

proteins binding sites and other related

substrate that the protein can anneal to

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Quick Re-cap

Osmosis: Waterdiffusion

• Osmosis- net diffusion of water across a membrane toward a higher solute concentration

• Definitions

– Water is a solvent

– Substances dissolved in a solvent (e.g. water) are solutes

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How it works

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Osmotic concentration

• When 2 solutions have different osmotic concentrations

– The hypertonic solution has a higher solute concentration ( the prefix “hyper” mean above or greater)

– The hypotonic solution has a lower solute concentration (the prefix “hypo” means below or less

• When two solutions have the same osmotic concentration

– the solutions are isotonic (the prefix “iso” means equal)

Osmotic pressure• Membrane strength determines

the effect on the cell

Human red blood cells

Plant cells

Maintaining osmotic balance• Some cells use extrusion

– water is ejected through contractile vacuoles

• Isosmotic regulation involves keeping cells isotonic with their environment– Water-dwelling organisms have developed ways to deal

with salt (marine) or water (fresh water) overload

– Terrestrial animals circulate isotonic fluid throughout their bodies

• Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid

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Active Transport

• Requires energy in the form of ATP

• Moves substances from low to high concentration

• Requires highly selective carrier proteins– Uniporters – move one molecule at a time

– Antiporters – move two molecules in opposite directions

– Symporters – move two molecules in the same direction

Types of active transports

Na+/K+ ATPase (Na+-pump)

• Uses ATP to drive Na and K transport against their concentration gradients (i.e. uphill)

• Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell

• ATP energy is used to change the conformation of the carrier protein

• Affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane

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Coupled transport

• Uses ATP indirectly (secondary active transport)

• Kinetic energy released when a molecule moves by diffusion supplies energy to transport a different molecule

• Symporter is used

• Glucose–Na+ symporter – energy from Na+ diffusion

drives glucose uptake into the cell

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Na+

K+

6. Dephosphorylation of protein triggers

change back to original conformation,

with low affinity for K+. K+ diffuses into

the cell, and the cycle repeats.

1. Carrier in membrane binds

intracellular sodium.

2. ATP phosphorylates protein with

bound sodium.

5. Binding of potassium causes

dephosphorylation of protein.

4. This conformation has higher affinity

for K+. Extracellular potassium binds

to exposed sites.

3. Phosphorylation causes

conformational change in protein,

reducing its affinity for Na+. The Na+

then diffuses out.

P

P

P

P

P

ATP

+ADP

Extracellular

Intracellular

Transportation by vesicle

• Endocytosis:– Phagocytosis

– Pinocytosis

Bulk or Vesicular Transport• Endocytosis: Movement of substances into the cell

– Phagocytosis – (phagein-to eat) cell takes in particulate matter

– Pinocytosis – (pino-to drink) cell takes in only fluid

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Receptor mediated endocytosis (coated pits)

• Exocytosis: Movement of substances out of cell

•

– Requires energy– Used in plants to export cell wall material– Used in animals to secrete hormones, neurotransmitters, digestive enzymes

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Bulk or Vesicular Transport

• Endocytosis: Movement of substances into the cell

– Receptor mediated endocytosis

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Bulk or Vesicular Transport

Exocytosis

What we’ve learned

• understand the role that cellular membranes,

concentration gradients and membrane proteins have on

cells exchanging molecules in their environment

• Principles of selectively permeable membrane

• Passive vs. Active Diffusion

• Channel vs. Carrier proteins

• Osmosis

• Na+/ K+ pump

• Coupled transport

• Bulk transport